What would cause a fuse to blow?

I have a 1978 280Z. Tail lights (driving) and side marker lights work fine until car is warmed up (about 20 minutes). After that the fuse blows. Before I start chasing wires I figured I'd ask incase someone has had this problem and can tell me the fix. All other lights (headlights, brakes) are fine. The turn signals don't work either (I've tried using different combo switches with the same results) but they are less of a problem then the back lights going out at night. Thanks

on my 78 280 i discovered that all the sockets for the tail lights, markers and directionals were a bit corroded and allowed them to short. take 'em apart, use some sandpaper, some deoxit, some dielectric grease and it'll do wonders. also, while you're in there check that the little center contact (the one on the spring plate) is seated correctly and not shorting to the outside cylinder - i had one of those as well.

Seems odd that the fuse blows when the engine warms up. Might just be coincidence. And you didn't say if the lights are on when the engine is warming up. You might leave everything on that would normally be on when the engine is running, except leave the engine dead. That will load the electrical circuit but take out any engine related functions. Although I can only think of one that might cause that. Maybe the floor temperature warning lamp, or its relay. If you have a catalytic converter.

I assume that the gauge lights go out also? On my 76 they're on the same circuit as the tails.

DeOxit seemed to work wonders on my car. I went through all the connectors behind the glove box, and the whole electrical system just got happier- the lights were brighter, the engine was smoother, etc.

I had a peculiar problem with one turn signal lately though. the bulb's contact point is rounded, and the socket's contact pad is sort of flat, and after hitting so many bumps, the turn signal would stop- or the two contact points would miss. So I just sanded the contact on the bulb to make it slightly flattened, and I had no problems since.

This probably isn't related, but the point is that the contact pads in the sockets can move around or 'grow' with corrosion, so it's possible that the vibrations from the engine could be causing a short to develop.

Could you try the procedure you mentioned with the rear lamp harness disconnected?

As others have stated. A few horrible connections will increase the resistance in the circuit. The engine probably has nothing to do with it. It is just takes a while for the resistance to build enough heat to blow the fuse. Start at the back and move forward.

As others have stated. A few horrible connections will increase the resistance in the circuit. The engine probably has nothing to do with it. It is just takes a while for the resistance to build enough heat to blow the fuse. Start at the back and move forward.

No. Increased resistance will not cause a fuse to blow.
Ohm's Law
V - Voltage
I - Current
R - Resistance
P - Power
Any way you cut it, increasing resistance will not increase current. Poor connections, such as the clamps holding the fuse, will cause hot spots. A prolonged hot spot in the fuse box will cause a fuse to fail and possibly cause the fuse box to melt.

A common problem with the Z cars is that corrosion can form on the sockets for the side markers (and front markers if the gasket is bad or the lens is cracked. As was mentioned, it can drop the resistance in that branch and possibly short out the fuse. The worst case scenario is that you have dropped the resistance, but not enough to be above the curve for the fuse. That, combined with the hot spot scenario described above can really heat things up.

I agree with Zed Head. The engine running is most likely a red herring. You probably won't have the parking lights on for very long if the engine isn't running. The first recommendation is probably the best. It might also be good to remove the fuse box and soak it in vinegar or a mild rust removing solution such as Evapo-Rust. After that, gently squeeze the clips together so they have better contact with the fuses.

Ok, so I checked the connectors and bulb plugs. I also checked the fuse box. Nothing looked out of place. All were very clean. I cleaned them anyways. Figured I didn't do much but I decided to let the car idle for about 35 minutes with the lights on. Went back outside after 35 and all the lights were still on.

Next test will be to drive at night and see if the fuse blows while driving since that is the only time I have had it happen.